But here's the critical part. In that verse it says 'whoever believes in Him'. That is the bit you have to do. If you do not believe in Him, how then can His personal sacrifice pay the price for your sin?
Now to believe in Him is more than just believing that He existed. There are historians who believe that Jesus existed because of all the historical documents that point to Him. But most of them don't believe in Him.
No; believing in Him, as other scriptures show, involves you acknowledging that you have sinned and believing that He died to pay the price that God required for your sin. Another scripture says that we should also believe that He also rose from the dead. Indeed, believing that He rose from the dead is a continuation of believing that He died for you. He rose from dying for you, so that you too will rise from the dead, rise to eternal life.
The Apostle Paul says this:
...if you will confess with your mouth the Lord Jesus, and believe in your heart that God has raised Him from the dead, you shall be saved.3